1. Field of the Invention
The present invention relates to an inverter control device for controlling voltage source inverters which drive an AC motor (hereinbelow simply referred to as a motor) such as an induction motor or synchronous motor.
2. Description of the Related Art
A known inverter control device of this type is illustrated in FIG. 1. Inverter 2 is provided in the main circuit system which drives motor 1. An inverter control device 10 is provided in order to control this inverter 2. A triangular wave generating unit 14 provided within this inverter control device 10 outputs a triangular wave in accordance with a triangular wave frequency which is set by a triangular wave frequency setting unit 13. Also, a voltage reference generating unit 21 outputs three phase voltage references Vu, Vv, Vw in accordance with an externally set current reference and the motor revolution frequency which is detected by a speed sensor 3 directly linked to motor 1. A PWM (Pulse Width Modulation) pulse generating unit 22 compares these voltage references Vu, Vv, Vw with the triangular wave output from triangular wave generating unit 14, to generate three phase PWM pulses VuPWM, VvPWM, and VwPWM, which are used to perform ON/OFF control of the switching devices (elements) constituting inverter 2. Thus, the prior art voltage source inverter shown in FIG. 1 is driven by PWM pulses under the control of a modulating frequency of fixed period.
However, in the case of large-capacity inverters such as for rail vehicles etc., if the switching frequency is made high in order to deal with problems such as generation of heat due to switching losses of the inverter, as the switching frequency is restricted to a frequency of 1 kHz.about.2 kHz, which is most unpleasant to human ears, electromagnetic noise from the motor produced noise which was unpleasant to passengers.
One method of achieving a reduction in perceived noise is the modulation method, in which the PWM modulation frequency is changed in irregular fashion instead of being kept at a constant value. With this method, the condition for obtaining a large benefit in terms of noise reduction is that the range from maximum to minimum of the modulation frequency should be made large; however, with a large-capacity inverter, there is a limit to the extent to which the maximum value of the modulation frequency can be increased, due to problems of switching loss. Also, if the minimum value of the modulation frequency is made small, if the fundamental frequency of the inverter was large (i.e. if the revolution frequency of the motor was high), this could cause deterioration of the current control characteristic, producing instability.